Process for producing coating on paper base having electrophotographic properties



United States Patent 3,472,673 PROCESS FOR PRODUCING COATING 0N PAPER BASE HAVING ELECTROPHOTO- GRAPHIC PROPERTIES Frederick W. Sanders, Chillicothe, Ohio, assignor to The gllfiad Corporation, Dayton, Ohio, a corporation of o No Drawing. Filed May 9, 1966, Ser. No. 548,429 Int. Cl. Htllb 1/06; D21h 5/00; B44d 1/44 U.S. Cl. 117-34 3 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to production of electrophotographic sheets of improved electrophotographic properties by the application to a base of an aqueous suspension of photoconductive material and malleable binder thereof, drying the resulting coating and subjecting the dried coating to calendering with permanent deformation of the coating without cracking.

The present invention relates to electrophotographic materials and process of producing same. More particularly, it relates to the use of resin emulsions as binders for photoconductive pigments.

An electrostatic printing process is one in which a visible record, reproduction or copy is produced and which includes the conversion of a light image into an electrostatic charge pattern. Examples of such electrostatic printing process include the xerographic process generally known in the trade as Xerox and a modification thereof generally known as Electrofax.

The Xerox process may be described generally as one where a base plate of relatively low electrical resistance such as metal, paper, etc. having a photoconductive insulating surface thereon is electrostatically charged in the dark. The charged coating is then exposed to a light image. The charges leak off rapidly to a base plate in any given area that is so exposed, after such exposure, the coating is contacted with electrostatic marking particles in the dark. These particles adhere to the areas where the electrostatic charges remain, forming a powder image corresponding to the electrostatic image. The powder image is then transferred to a sheet of transfer material resulting in a positive or negative print, as the case may be. A detailed description of the steps of such a process is found in United States Patent 2,297,691 issued Oct. 6, 1942 to C. F. Carlson.

Alternatively, where the base plate is relatively inexpensive as of paper, is may be desirable to fix the power directly'to the plate itself. In such case, the process is generally known as Electrofax whereby direct electrophotographic printing is effected on paper. The Electrofax process may be described generally as including the following steps for making a direct print. The paper is first made sensitive to light by giving it a blanket negative electrostatic charge on the coating side in the dark. One way of doing this is by ion transfer from a corona discharge. The sheet, now sensitive to light, is exposed by any of the conventional photographic procedures. The electrostatic charge is lost or reduced in the exposed area and retained in the masked areas to form a latent electrostatic charge image on the surface of the paper. The latent image is then developed by applying a pigmented resin powder carrying a positive electrostatic charge. The powder is attracted and held by the megatively charged image areas. Finally, the powder image is fixed by melting the resin powder so it fuses to the papersurface to produce a durable, lightfast image. A detailed description of the steps of such a process is 3,472,673 Patented Oct. 14, 1969 given by H. G. Greig in U.S. Patent 3,052,539 issued Sept. 4, 1962.

The latter process eliminates the intermediate step of transferring the powder image and enables production of the image directly upon a desired surface. A photoconductive paper suitable for use in connection with such a process usually comprises a backing sheet, such as paper, coated with photoconductive particles such as zince oxide, zinc sulfide, cadmium sulfide, titanium sulfide, etc., suspended in an electrically insulating filmforming binder. A photoconductive printing base, such as zinc oxide, while satisfactory for carrying out the Electrofax process, is also subject to certain limitations, many of which are primarily physical in character.

The electrically insulating film forming material may be one of a number of substances such as various synthetic resinous materials having high dielectric strength. Previously used materials of this type include polyvinyl acetate,, copolymers of vinyl chloride-vinyl acetate, polystyrene, silicone resins, etc., as well as resin-like materials such as methyl or ethyl cellulose and cellulose nitrate, natural resins and waxes.

Th photoconductive powder may be incorporated into the selected film-forming material in various ways. For example, the selected film-forming material may be dissolved in an organic solvent and then mixed with the zinc oxide or the photoconductive agent may be kneaded dry with the film-forming material and heated to a sufiiciently high temperature to make it plastic. Other methods may also be suitably employed, including the use of aqueous dispersions of the zinc oxide or other photoconductive material.

The dispersion of the photoconductive material in the selected dispersion medium is then applied to the base in any suitable manner as, for example, by immersing the base in the dispersion or suspension of photoconductive film-forming composition and then removing the water or solvent by evaporation, or by spraying or otherwise coating one or both sides of the base with the dispersion and then evaporating the liquid of the dispersion. Other suitable methods may also be used.

It has long been known that aqueous dispersion systems have certain advantages over solvent systems, including that of not requiring additional recovery equipment for the solvent, and prevention of fires and explosions. However, although the use of water dispersions has generally wider applications in industry, they are subject to certain disadvantages in that many of the dispersions or emulsions using certain binders are found to be ineffective electrophotostatically, or else are physically incapable of admixture with the photoconductive agent because of precipitation or gellation of materials and excessive conductivity, or else lack of suitable adhesion.

Although it has previously been found that certain of these otherwise ineffective resinous binder are materially improved by employing shellac as the dispersing agent, in that the discharge characteristics are improved, thus making them useful in electrophotography, other resins showed, under similar conditions, very little improvement other than mechanical stability.

It has now been found, in accordance with the present invention, that certain resin emulsions which cannot be used effectively as binders for electrophotographic compositions, can be elfectively used by calendering the dried films of such com-positions.

It is, accordingly, an object of the present invention to provide -a method of making effective use of resin emulsions effective as binders in electrophotography which have not previously been commercially satisfactory for this purpose.

Other objects of the invention will be evident from the description which follows.

Many film-forming materials used in production of electrophotographic sheets serve as effective binders for the photoconductive agent when applied to the base or sheet of paper or other material in proper physical form. Until, however, they reach this form, they are relatively ineffective in that a proper bonding of the photoconductive agent to itself and to the base is not obtained. Even when applied to the base in dispersed form, certain film-forming agents deposit in the form of distinct particles or droplets, visible under the microscope, which do not readily coalesce to form the'continuous film necessary for good bonding. With some film-forming materials, the incorporation of a plasticizer for the particular resinous materials as, for example, tricresyl phosphate, tributyl phosphate, etc., gives sufficient fiow to enable the particles or droplets to form a continuous film. With other materials, however, the incorporation of a plasticizer still does not give a satisfactorily adhesive film. While the plasticizer may improve the flow of such a resin somewhat, the latter still does not readily and quickly flow out to a continuous film from the particles or the individual droplets formed from the applied dispersion, unless excessive amounts of plasticizer are used.

An example of resinous binder which shows little utility in making electrophotographic compositions, other than in mechanical stability, even when used in connection with shellac as a dispersing agent is the extremely small particle size latex comprising 60% styrene and 40% butadiene available under the name of X33 13 manufactured by Dow Chemical Company. This product, however, was found to function quite satisfactorily as a binder for photoconductive powders and to produce excellent electrostatic photographs after calendering the dried film between heavy rolls to coalesce the individual small particles into the form of a continuous film. This desirable result was found with other resin emulsions provided the resins were sufficiently malleable to be calendered with permanent deformation without cracking. For example, one of such resin emulsions is a terpolymer latex of 60% vinyl acetate, 38% 2-ethy1 hexyl acrylate and 2% acrylic acid, and is manufactured and sold by Borden Company under the name Polyco 134.

The use of a very hard, high molecular weight polyvinyl acetate resin, known in the trade as Duracet M manufactured and sold by Franklin Chemical Company, did not provide the desired image attainable by this invention. However, the addition of 5 to 15% tri-cresyl-phosphate increased the malleability of this polyvinyl lactate resin so that following calendering, a highly satisfactory image was attained. It has also been found that plasticized polystyrene, ethyl-cellulose and methyl methacrylate resin emulsions are suitable as binders of this invention.

The flow of resin particles apparently causes compacting and improvement of the continuity of the resistance coating, thereby improving the uniformity of the chargeholding capacity in the dark. Results similar to those obtained by calendering have been obtained by heating the dried films at elevated temperatures. However, more effective results are obtained in a shorter time and more simply by calendering, where satisfactory results are not obtained without added treatment.

The calendering is preferably effected at 600-1800 pounds per linear inch for 1 to 7 nips. Satisfactory results were obtained with both steel rolls, as in the usual machine calender, and with the so-called cloth or paper roll and steel of the supercalender.

Calendering has been found to improve the electrophotographic properties of photoconductive agents dispersed in film-forming materials which are deposited on the base in the form of discrete particles which do not readily and quickly coalesce into a continuous film and the process of the present invention is intended to apply only to such type materials. C-alendering of films formed from materials which either do not form as discrete particles, or which form as discrete particles which readily and quickly coalesce is generally unnecessary. Calendering has been found to be effective with photoconductive films wherein the ratio of binder to photoconductive agent is between 1 to 2 to 1 to 8.

The particular plasticizer most desirable for use is that which serves as the most effective plasticizer for the particular film-forming material used, provided the dielectric properties of the film forming material are not degraded. It is customary to use plasticizers with certain film-forming agents which give hard films which tend to crack when bent or upon use under low temperature conditions. Such plasticizers are suitable for use in the present invention. In general, any suitable plasticizer is suitable which, in the amounts used, does not make the final film tacky or too soft under conditions of use.

What is claimed is:

1. In a process for producing at high rates of speed coatings on a paper base in the form of continuous films having improved electrophotographic properties including uniform discharge characteristics by the deposit on said paper base of a dried coating from an aqueous suspension comprising finely divided photoconductive pigments and water-insoluble electrically insulating resin binder in dispersed particulate form and which is sufficiently malleable to be callendered with permanent deformation without cracking, the improvement which comprises calendering at 600-1800 pounds per linear foot for 1-7 nips the dried coating comprising 1-8 parts by weight of said pigment and 1-2 parts by weight of said binder.

2. The process of claim 1 wherein the dried coating contains a plasticizer of said binder.

3. The process of claim 1, wherein said binder is selected from the group consisting of high molecular weight polyvinyl acetates, copolymers of styrene and butadiene, terpolymers of vinyl acetate, 2-ethyl hexyl acrylate and acrylic acid, plasticized ethyl cellulose, plasticized polystyrene, and plasticized methyl methacrylate.

References Cited FOREIGN PATENTS 594,654 3/1960 Canada.

WILLIAM D. MARTIN, Primary Examiner WILLIAM R. TRENOR, Assistant Examiner U.S. Cl. X.R. 96-75; 117-652 

